Highly suggest people look into the actual pages if they have any questions/concerns.
For anyone unsure or confused, the one trillion trees site isn't saying they are going to plant all 1 trillion trees themselves — it's more like an aggregator for other organizations. The only one that the site seems actually affiliated with is the Yucatán one, and that one is $1 per tree. A quick search says they plant 8 different native species in that region.
The one most people have been donating to — the $0.10 per tree Eden Project — if you check the org's actual website, you'll see they actually give a range for what the trees cost. And what they plant depends on the location. For example, in Madagascar it says they started by planting one type of tree. By 2012, they had expanded to several varieties of native trees, they have local workers harvest the seeds from the remainder of the existing forests. But saying $10 can plant 100 trees is obviously a great hook so can you blame them for keeping it simple?
Many of these organizations also have other purposes, such as improving the livelihoods of the local population. For example, in Haiti, where people are still struggling to recover from ecological issues and extreme poverty, it says they plant mostly agroforestry trees so the people can make a living and also to help with food security; in this case a smaller proportion of the trees planted are for native reforestation.
I saw one organization that wanted to plant coffee trees; I saw another that included bee-keeping. Many have clean water concerns. I understand that there is a "best" or better way to reforest areas. If anyone has specific concerns about any of the orgs on the site, I'm sure all of us would be grateful to hear it.
I'll kick off with one — I didn't donate to ForestNation because this is what they say they do: ForestNation is a for-profit social business that provides corporations turnkey environmental impact marketing collateral to benefit their bottom line. Their main aim is apparently to "re-connect us with each other and nature" via planting trees or something. Which is fine, I guess, but not what I was looking for. My point is, this information is out there. Pick one that works for you.